Compare with Lactuca serriola, Prickly Lettuce. Here is a link that might be useful:

Solanum carolinense perhaps? HTH Chris Here is a link that might be useful: Carolina horsenettle

As I research more on the suggestion by floral_uk z.8/9 SW UK regarding Talinum plants, I've discovered that this plant may be the Talinum fruticosum (T. triangulare). Although all the pictures I've seen show that the Talinum fruticosum has purple flowers, this species can also have yellow, pink, red, or white flowers. So I believe this plant is indeed the Talinum fruticosum (T. triangulare). It is actually eaten as a vegetable in some countries, maybe one day I'll nibble on one of its leaves. Thanks very much.

Could you tell us where this is growing geographically? Also can you provide a photo of the whole plant so we can assess the habit and one of the lower leaves? Plus an indication of the flower size. There are several Sow Thistkes, Sonchus species. I’m not yet sure that this is one of them.